It has been found desirable to have a high-power index-guided diode laser operating in a wavelength-locked single longitudinal mode and with a well defined single lateral spatial mode. Such diode lasers have applications in space communications, optical recording and fiber-optics. To achieve such a diode laser, a channeled substrate planar (CSP) AlGaAs diode laser was developed. Such a diode laser has a channel in the substrate for lateral confinement and a clad layer over the substrate and filling the channel. The clad layer has a planar surface over which the active layer of the diode laser is grown epitaxially. This type of diode laser has all of the above desired properties except spectral stabilization.
In order to achieve both spectral and spatial stability, a channel substrate, planar, distributed-feedback diode laser was developed. Such a diode laser is described in an article entitled "Efficient AlGaAs channeled-substrate-planar-distributed feedback laser" by B. Goldstein et al., APPLIED PHYSICS LETTERS, Vol. 53(7), Aug. 15, 1988, pages 550-552. This diode laser includes a grating to achieve spectral stability. The grating shown in this article is an interrupted grating in that it is positioned in the wings or shoulders of the device on either side of a V-channel in the substrate. Although the structure displays distributed feedback characteristics and the desired spatial stabilization, the devices exhibit distributed feedback behavior only over a limited temperature range (&lt;10.degree. C.) and the yield of the devices having these desired characteristics is very low (less than about 5%). The reason for this is that the optical mode only couples to the grating over the wings or shoulders of the device and thereby provides a very low coupling coefficient, which describes the extent of the interaction between the optical field of the diode laser and the grating.
To enhance the interaction between the grating and the optical field, it has been suggested that the grating be positioned in the structure such that a greater portion of the optical mode interacts with the grating. Diode lasers having confined channels with a grating extending across the entire device are shown in U.S. Pat. No. 4,257,011 (M. Nakamura et al.), issued Mar. 17, 1981, and entitled "Semiconductor Laser Diode" and in an article entitled "Continuous room-temperature operation of a 759-nm GaAlAs distributed feedback laser" by S. Takigawa et al., APPLIED PHYSICS LETTERS, Vol. 51(20), Nov. 16, 1987, pages 1580-1581. Although this structure greatly increases the coupling coefficient, it also provides a large beam divergence of the emitted beam (as large as 50.degree.). Such a beam divergence is undesirable for most applications requiring coupling to optical systems. A smaller beam divergence in the range of about 25.degree. to 29.degree. allows for more efficient use of the light output from the diode laser with a simple optical system.
It is desirable to have a semiconductor diode laser which not only has a high coupling coefficient for larger temperature ranges of stable single spatial and longitudinal mode operation and high yield, but also has a small beam divergence.